The main problem is that anyone who can do both is highly incentivized to go into engineering/coding instead of product design. The pay and respect are usually much better. So then that pool is limited further to people who can design and code well, prefer design, and don't need/want extra money.

Then add on to that our current work culture prefers specialists to generalists: HR and exec teams will hire a product designer with 5 years of design experience and no coding over someone with 2 years of design experience and 5 years of software engineering experience.